You should have a good idea of the kind of machine you want to buy before you walk into the store or pick up the phone. If you're confident about the model you need, you may want to consider mail order. Salespeople may not be knowledgable or may try to talk you into a more expensive model, so be prepared to stick to your choice. But before you buy, confirm that the fax is new, not refurbished, and that it's covered under warranty.
Office superstores offer competitive prices (although the selection can be quite limited). These stores tend to cater to customers with problems or returns, so don't expect to get expert advice. Since the salespeople deal with so many other products, their level of specialized knowledge is often low.
Before taking a machine home, get some hands-on experience. Check its noise level, the space it takes up, and how the paper is loaded and output. You may never pick up the user manual, so it had better be easy to operate.
Should you purchase a service contract? Probably not. Fax machines are among the least likely of all office equipment to break or malfunction. And among the malfunctions, the most common cause is user error. As long as you use it properly and teach anyone else who might operate it to do the same, you should have few problems with your machine.